THE BONES. 73 



The largeness of the canal accommodates itself to the mobility of the par- 

 ticular regions of the vertebral column, and to those portions of the spinal 

 cord whence the greatest number of nerve filaments passes oif. In the cervical 

 and lumbar regions the column is most moveable, the nerves forming the 

 Brachial and Lumbar plexuses very numerous, the canal the widest ; in the 

 dorsal and pelvic portions the reverse. 



The anterior wall of the canal is formed by the vertebral bodies lying 

 close over one another, merely separated by the intervertebral ligaments, the 

 posterior by the vertebral arches, the interspaces between which are closed 

 by short extensible ligamentous masses, ligament, flava. 



On both sides the furam. intervertebralia open into it. 



Canulis vertebralis, vertebral canal, on either side of the neck is formed by 

 the over-lying fnram. vertebralia of the tran^vtrpe processes ; it contains urteria 

 et vena vertebralis. 



81. The Tissue of Vertebral Bodies, 



consists almost entirely of spongy substance with large meshes, covered ex- 

 ternally with a thin layer of compact tissue. A great portion of the processes, 

 and the whole of the arches, consist of the last mentioned. 



The vertebrae are very richly supplied with blood-vessels. For this pur- 

 pose we find on the dorsal surface of each vertebral body a horizontal canal, 

 which divides into several and always smaller branches in the spongy sub- 

 stance, all of which are lined with a thin layer of compact bone and per- 

 forated by innumerable smaller foramina. 



82. Development of the Vertebral Column. 



In the first months of fetal life the vertebral column forms the entire 

 length of the trunk, at birth three-fifths, and in the adult only two-fifths of 

 the length of the body. 



In the first period of development the curvatures of the Vertebral column 

 are wanting, later it becomes curved in the form of a wavy line, the convex 

 surface in the neck is directed forwards, in the dorsal region backwards, in 

 the lumbar region, again, forwards, and in the pelvic strongly backwards. 

 The smallest deviation from the curvature in one of the above named parts 

 produces corresponding deviations in the other parts. 



Besides these there exists in the region of the 4th and 5th dorsal vertebrae, 

 in the region of the Aortic arch, a lateral inclination ; the concavity to the left, 

 convexity to the right. 



Abnormal curvatures, forwards, backwards, and laterally by wasting or de- 

 struction of the vertebrae, or by inactivity of the muscles of one side and pre- 

 ponderating activity of those of the other, occur. 



In old age the vertebral column curved forwards frequently exhibits a 

 union of several dorsal or lumbar vertebrae. 



The height of the column differs according to age, increasing to about the 

 twenty-fifth year, and decreasing upon the advance of old age. Its average 

 vertical diameter measures 2 feet 2 inches. 



The separate vertebrae are developed from three centres of ossification, at 

 first on the lateral boundaries of the spinal canal, as a defence for the spinal 

 cord, and vertebral tody. Five other points of ossification are presented, only 

 later, for the processes. 



